Metal and ceramic powder injection molding is a low cost way to produce complex and precision-shaped parts from a variety of materials. It is common for this process to produce equivalent parts for 50% or less of the cost of conventional machining or casting. Total savings are a function of shape complexity, production volumes and overall size of the part.
There are four primary steps to producing powder injection molded parts or articles:
1. Feedstock Formulation: Very fine, usually less than 25 microns, elemental or prealloyed metal or ceramic powders are hot mixed with a polymeric binder. This mixture is then cooled and granulated to form the feedstock for an injection molding machine. PA1 2. Molding: Molding is done in an especially equipped plastic injection molding machine modified to mold the powder/polymer mixture into an oversized shape of powder and binder called a "green part" or a "green preform". By virtue of powder injection molding, intricate detailed features can be molded into the oversized green preform, including threads, holes, radii, contoured surfaces, logos and text. The molding process produces the same shape every time, providing uniformity from part to part. Furthermore, the process produces virtually no waste. PA1 3. Debinding: Seventy-five percent to ninety percent of the binder material is removed from the green part. Several different binder removal methods are used depending on the chemical and physical properties of the binder formulation used. These include thermal debinding, where a thermoplastic binder is baked out in an oven using elevated temperatures; solvent debinding, where the binder is dissolved using a chemical or water; and catalytic debinding, where the binder is reacted out by the introduction of a catalyst. After the binder is removed, the resulting object is called a "brown part" or "brown preform". It consists of a porous matrix of metal powder and a small amount of binder, just sufficient to allow the part to retain its shape and hold together. PA1 4. Sintering: In the final step, the brown preform is sintered in a furnace or oven through a complex profile of temperatures, pressures and/or atmospheres depending on the material being processed and the physical properties desired. A microprocessor controls these variables, ultimately bringing the part to within 96% of its melting point. At the lower temperatures of the sintering cycle, the residual binder is removed. As the temperatures increase, neck growth between powder particles begins, bringing the particles together and reducing porosity. The higher temperatures of the sintering profile continue this trend, ultimately densifying the metal to approximately 98%. Densification results in shrinkage of 14% to 25% depending on the solids loading of the feedstock and the alloy. This shrinkage is predictable and compensated for by oversizing the green part mold cavity by the precise amount.
Powder injection molded parts and articles are normally produced to finished dimensions in these four steps. The process is an innovative way to product complex shaped articles with consistency and accuracy. A variety of no materials can be processed, including stainless steels and other materials that may be difficult to form in other ways. The process provides design flexibility and delivers tolerances of +/-0.002 to 0.003 inches per inch. Because the parts are sintered essentially to "full density", the parts produced by the process have properties which are virtually the same as those as wrought materials.
One aspect of the process that poses particular problems and hazards is the debinding step. Sufficient binder must be retained to provide a brown part that is stable and sufficiently strong to be handled and transported between the debinding and sintering steps, but the brown preform should not contain either a type of binder or an amount of binder that would hinder or impair the sintering step. Binding systems currently employed include a wax and polymer system which is thermally debound; a water soluble/cross linkable binder available from Thermat Precision Technology, Inc. under the trademark "Thermat Pristine" which is partly debound in water, cross-linked and thermally debound; a water and agar based binder available from Allied Signal; and an acetal based system available from BASF A.G. which is debound in a gaseous acid-containing atmosphere. The temperature and/or the duration of the debinding step in all of these systems is quite critical.
The BASF acetal-based system, which is described in U.S. Pat. No. 5,362,791 issued Nov. 8, 1994 to Ebenhoech et al. and other publications, has found particular application in the powder injection molding of stainless steel. In this system the polymer binder for the powder is polyoxymethylene and the acid employed for debinding the polymer is anhydrous nitric acid, which is an extremely hazardous substance creating serious environmental concerns.